The invention relates to an apparatus and method for generating electronic signals for use in the field of seafloor electromagnetic exploration.
Determining the response of the sub-surface strata within the earth's crust to electromagnetic fields is a valuable tool in the field of geophysical research. The geological processes occurring in thermally, hydrothermally or magmatically active regions can be studied, for example. In addition, electromagnetic sounding techniques can provide valuable insights in to the nature, and particularly the likely hydrocarbon content, of subterranean reservoirs in the context of subterranean oil exploration and surveying.
Seismic techniques are often used during oil-exploration expeditions to identify the existence, location and extent of reservoirs in subterranean rock strata. Whilst seismic surveying is able to identify such voids the technique is often unable to distinguish between the different possible contents within them, especially for void contents which have similar mechanical properties. In the field of oil exploration, it is necessary to determine whether a previously identified reservoir contains oil or just seawater. To do this, an exploratory well is drilled to sample the contents of the reservoir. However, this is an expensive process, and one which provides no guarantee of reward.
Whilst oil-filled and water-filled reservoirs are mechanically similar, they do possess significantly different electrical properties and these provide for the possibility of electromagnetic based discrimination testing. A known technique for electromagnetic probing of subterranean rock strata is the passive magneto-telluric (MT) method. The signal measured by a surface-based electromagnetic detector in response to EM fields generated naturally, such as within the earth's atmosphere, can provide details about the surrounding subterranean rock strata. In practice a series of detectors are used to isolate effects which are local to each detector. However, for deep-sea surveys, all but those MT signals with periods corresponding to several cycles per hour are screened from the seafloor by the highly conductive seawater. Whilst the long wavelength signals which do penetrate to the sea-floor can be used for large scale undersea probing, they do not provide sufficient spatial resolution to examine the electrical properties of the typically relatively small scale subterranean reservoirs.
To overcome the lack of suitable MT signals at the seafloor, active EM sounding can be employed. Information about the subterranean strata is determined by examining the response of remote detectors to an artificial EM source, where both the detectors and source are located at, or near, the seafloor. Benefits of this method include the ability to know a priori the input signal to which the subterranean rock strata are exposed, the ability to select particular frequencies and coherence lengths of EM signal and the ability to set the relative geometry of transmitter and receiver antennae.
FIG. 1 of the accompanying drawing shows schematically how a surface vessel 14 undertakes EM sounding of the subterranean rock strata 8 in which a reservoir 12 has already been identified. The surface vessel 14 floats on the surface 2 of the sea 4. A deep-sea vessel 18 is attached to the surface vessel 14 by an umbilical connector 16 which provides an electrical, optical and mechanical connection between the deep-sea vessel 18 and the surface vessel 14. The deep-sea vessel 18 is towed by the surface vessel 14 such that it remains consistently close to the seafloor 6. This is facilitated by an echo-location package 20 which relays information about the height of the deep-sea vessel 18 above the seafloor 6 to the surface vessel 14. The deep-sea vessel 18 receives electrical power from the ship's on-board power supply via the umbilical 16. A cycloconverter unit 30 generates the chosen waveform to be supplied to a transmitting antenna 22. The antenna 22 is towed by the deep-sea vessel 18. The antenna 22 broadcasts the EM signal into the sea 4, and this results in a component passing through the rock strata 8. A remote instrument package 26 records the signal received by an antenna 24 in response to the transmitted EM signal. If the separation of the transmitting antenna 22 and the receiving antenna 24 is greater than a few hundred meters, the highly conductive seawater strongly attenuates the direct signal between them. The components of the EM signal that have travelled through the rock strata 8 and the reservoir 12 dominate the received signal and provide information about the electrical properties of these regions. At the end of the sounding experiment, a remotely deployable flotation device 28 carries the instrument package to the surface 2 for recovery and retrieval of data for inversion analysis.
FIG. 2 of the accompanying drawings schematically shows the deep-sea vessel 18 and transmitting antenna 22 in more detail. The umbilical 16 is attached to the deep-sea vessel 18 via a swivelable connection 32. The echo-location package 20 and cycloconverter unit 30 are carried within the deep-sea vessel 18. A fin 34 helps to stabilise the deep-sea vessel 18 as it is drawn through the seawater 4. The antenna 22 is attached to the deep-sea vessel 18 by a towing bar 36. The antenna 22 comprises a fore electrode 38 and an aft electrode 42. The EM signal generated by the cycloconverter unit 30 is applied to the electrodes 38, 42 via signal cables 40, 44. It is the conducting seawater 4 which provides the unscreened return path for the electrical current and generates a dipolar EM field. The electrodes 38, 42 and cables 40, 44 are supported by a neutrally buoyant hose 46. A tail rope 48 supplies a drag force to the trailing end of the antenna 22 to assist in keeping it correctly extended.
The exact choice of the waveform supplied to the transmitting antenna 22 and the ability to vary its fundamental properties, such as frequency, is important. Different frequencies of EM signal will propagate differently through the rock strata 8. Each frequency therefore provides information which is sensitive to the particular conditions along different paths within the rock strata 8, and together allow for more detailed mapping. The stability of the waveform in amplitude, frequency and phase are crucial to providing the best possible examination of the rock strata 8. For example, with no direct connection between the transmitting 22 and receiving 24 antennae, it is impossible to transmit information about phase drifts in the source EM signal to the instrument package 26. Accordingly, it is impossible to distinguish between a drift in the phase of the source signal and a change in the propagation time between source and receiver. The precision to which the electrical properties of the rock strata, which determine the signal time delay, can be determined is therefore highly dependent on the stability of the source signal, the generation of which is not a straightforward task.
The requirement to transmit power at a level of several kilowatts through the umbilical 16 necessitates the use of a relatively high voltage, low current supply in order to minimnise transmission losses. However, such an a.c. power source has significantly different characteristics from those desired for the outgoing waveform.
It is the purpose of the cycloconverter unit 30 is to transform the input a.c. power supply (high voltage, low current, fixed frequency sinusoid) into the desired transmitter waveform (low voltage, high current, variable and controllable frequency and waveform).
One way of generating an output signal of the desired frequency from the input signal is through a half-wave rectifying bridge circuit that is controllably switchable at the zero crossings of the input signal.
FIG. 3 is a graph which schematically represents an ideal output signal from such a bridge, which has reduced the frequency of the output signal by a factor of 5 by switching the bridge at every fifth zero crossing of the signal at the input frequency.
FIG. 4 of the accompanying drawings schematically shows an ideal 256 Hz input voltage as a function of time. The switching takes place on zero crossings of the input waveform (marked with bold vertical lines in the figure). The control operates by detecting these zero crossings, and immediately switching the bridge to provide the appropriate polarity of output for the next half cycle. The frequency and phase of the transmitter output signal depends on the timing at which the polarity of the output half cycles is changed. This can be controlled in two ways.
One approach would be to rely on using a frequency stabillised power supply from the surface vessel to the transmitter's cycloconverter unit. Control over both the frequency and phase of the output signal can in principal be achieved by controlling the phase and frequency stability of the power supply. However, such an approach faces technical problems caused by the capacitive and inductive effects in the tow cable, the cycloconverter itself, and the dipole transmitting antenna, as now being explained.
The tow cable may be constructed using either co-axial or spiral wound electrical conductors. In either case, and especially in the case of a co-axial construction tow cable, several kilometres of cable constitute a very significant capacitance between the power source and the deep tow vehicle. Typically the cable also has some inductance; but the transmission characteristics will vary from cable to cable, and to a lesser extent will also depend on the relative amounts of the cable that are immersed in sea water or wound onto the drum of the towing winch.
The transmitting dipole antenna must be designed to have as low a resistance as possible, in order to optimise the transmitter dipole moment for a given power level. It will however have a significant self-inductance, which will to some extent depend on the characteristics of the seawater through which it is being towed and its proximity to, and the properties of, the seafloor.
There are two major effects of the capacitive and inductive properties of these components of the transmitter system. First, in general the current at any point is not in phase with the voltage. Second, even if the power supply at the surface vessel is designed for low harmonic distortion, the input voltage and current waveforms at the deep-tow vessel are significantly affected by higher harmonics of the supply frequency and by standing waves set up in the system between the ship board power supply and the transmitting electrodes in the antenna. The exact properties of these harmonics and standing waves are difficult to predict, and are likely to vary significantly between installations on different vessels, and even within a single deployment of the transmitter system as tow cable is paid out and hauled in.
FIG. 5 of the accompanying drawings schematically shows how the ideal 256 Hz input voltage indicated in FIG. 4 might more realistically appear due to the effects described above. In response to the input waveform indicated in the figure, the cycloconverter control is liable to detect spurious and unpredictable zero crossings which are due to harmonics or standing waves, and not to the fundamental supply waveform. This is apparent from the number of crossings, again marked with bold vertical lines in the figure. If the zero crossings are used to control the switching of individual half sinusoids, and to control the timing of output waveform polarity reversals, then the source becomes subject to unpredictable frequency jitter and phase drift.